4 ms·

It’s a process of transition between two distinct states. Say a bit flips from 0 to 1; there is a short time period during which that bit is physically “in tran

by trevyn 7y ago

It’s a process of transition between two distinct states. Say a bit flips from 0 to 1; there is a short time period during which that bit is physically “in transition”, but it would be incorrect to say that the bit has a value of, say, 0.5 during the transition. Also remember that quantum mechanics is a way to make statistical predictions of outcomes of certain experiments; it does not claim to explain what is actually happening underneath.

joycian 7y ago
With your second paragraph firmly in the back of my mind: In the sister comment thread, two photon excitation seems to indicate that this intermediate state (0.5 in your coin example) is not invalid. Can you help me understand better? Edit: Also, I think your coin "in transition" implicitly assumes that the system has more degrees of freedom than 1 bit (You can only flip a coin in 3 dimensions).
gaze 7y ago
"it would be incorrect to say that the bit has a value of, say, 0.5 during the transition." In what sense? It has a <z>=0, meaning there's a statistically 50%/50% chance of measuring 1 or 0 but the system is in principle coherent. This means that half way through the jump, if you rotated it 90 degrees, you'd measure a _definite_ z value of -1 or 1, or you'd measure a bit value of 0 or 1, depending on whether you twisted it clockwise or counterclockwise.
- wnoise 7y ago
  In the sense that it ignores that coherence. Any state on the disc of the Bloch sphere's equator has <z> = 0, and 50:50 chance of measuring 1 or 0. If you want to assign a real value of 0.5 to a bit, then the only one that makes sense is the center, completely decoherent mixture.
DiogenesKynikos 7y ago
I strongly disagree with your second paragraph. Quantum theories do, we think, correspond to what is actually happening underneath. They might not be perfect correspondences, in that our theories are incomplete or approximate, in the same way that Newtonian gravity is an approximation of General Relativity in the weak-gravity regime. Bell's inequality places a strong constraint on what sort of physical theories can explain quantum phenomena. If you believe in locality (the universe has no global variables, and information propagates outwards through local interactions), then the wavefunction is a real thing, and the actual state of the universe.
- trevyn 7y ago
  The formalisms of quantum mechanics are widely accepted, but as you know, there is considerable disagreement about what may actually be happening underneath: https://en.m.wikipedia.org/wiki/Interpretations_of_quantum_mechanics https://en.m.wikipedia.org/wiki/Interpretations_of_quantum_m... (Aside: Bell’s theorem additionally requires one to dismiss the possibility of superdeterminism.)
  - joycian 7y ago
    So this is what I do not understand about Bell's Theorem: it seems to dismiss in the assumptions the possibility of determinism (I don't understand where the super comes from, as there is no distinction) and then goes on to conclude "Hey, there must be fundamental randomness". Didn't we just assume the conclusion?
    - DiogenesKynikos 7y ago
      It does not assume from the outset that there is no determinism. It sets bounds on the types of results that you can get with deterministic local variables. Those bounds are violated in the real world. Hence, the universe is either quantum mechanical, or there exist hidden nonlocal variables that give the illusion of quantum behavior.
      - trevyn 7y ago
        Bell: There is a way to escape the inference of superluminal speeds and spooky action at a distance. But it involves absolute determinism in the universe, the complete absence of free will. Suppose the world is super-deterministic, with not just inanimate nature running on behind-the-scenes clockwork, but with our behavior, including our belief that we are free to choose to do one experiment rather than another, absolutely predetermined, including the "decision" by the experimenter to carry out one set of measurements rather than another, the difficulty disappears. There is no need for a faster than light signal to tell particle A what measurement has been carried out on particle B, because the universe, including particle A, already "knows" what that measurement, and its outcome, will be. https://en.m.wikipedia.org/wiki/Superdeterminism https://en.m.wikipedia.org/wiki/Superdeterminism I think it’s useful to be clear about the dividing line between demonstrated scientific results and our scientific intuitions, even when those intuitions are driven by observed patterns that have been reliable in the past. Intuitions are excellent drivers for formulating new theories and experiments, but it is epistemically dangerous to conflate beliefs and knowledge in our minds.

by trevyn 7y ago

joycian 7y ago

With your second paragraph firmly in the back of my mind: In the sister comment thread, two photon excitation seems to indicate that this intermediate state (0.5 in your coin example) is not invalid. Can you help me understand better? Edit: Also, I think your coin "in transition" implicitly assumes that the system has more degrees of freedom than 1 bit (You can only flip a coin in 3 dimensions).

gaze 7y ago

"it would be incorrect to say that the bit has a value of, say, 0.5 during the transition." In what sense? It has a <z>=0, meaning there's a statistically 50%/50% chance of measuring 1 or 0 but the system is in principle coherent. This means that half way through the jump, if you rotated it 90 degrees, you'd measure a _definite_ z value of -1 or 1, or you'd measure a bit value of 0 or 1, depending on whether you twisted it clockwise or counterclockwise.

wnoise 7y ago
In the sense that it ignores that coherence. Any state on the disc of the Bloch sphere's equator has <z> = 0, and 50:50 chance of measuring 1 or 0. If you want to assign a real value of 0.5 to a bit, then the only one that makes sense is the center, completely decoherent mixture.

DiogenesKynikos 7y ago

I strongly disagree with your second paragraph. Quantum theories do, we think, correspond to what is actually happening underneath. They might not be perfect correspondences, in that our theories are incomplete or approximate, in the same way that Newtonian gravity is an approximation of General Relativity in the weak-gravity regime. Bell's inequality places a strong constraint on what sort of physical theories can explain quantum phenomena. If you believe in locality (the universe has no global variables, and information propagates outwards through local interactions), then the wavefunction is a real thing, and the actual state of the universe.

trevyn 7y ago
The formalisms of quantum mechanics are widely accepted, but as you know, there is considerable disagreement about what may actually be happening underneath: https://en.m.wikipedia.org/wiki/Interpretations_of_quantum_mechanics https://en.m.wikipedia.org/wiki/Interpretations_of_quantum_m... (Aside: Bell’s theorem additionally requires one to dismiss the possibility of superdeterminism.)
- joycian 7y ago
  So this is what I do not understand about Bell's Theorem: it seems to dismiss in the assumptions the possibility of determinism (I don't understand where the super comes from, as there is no distinction) and then goes on to conclude "Hey, there must be fundamental randomness". Didn't we just assume the conclusion?
  - DiogenesKynikos 7y ago
    It does not assume from the outset that there is no determinism. It sets bounds on the types of results that you can get with deterministic local variables. Those bounds are violated in the real world. Hence, the universe is either quantum mechanical, or there exist hidden nonlocal variables that give the illusion of quantum behavior.
    - trevyn 7y ago
      Bell: There is a way to escape the inference of superluminal speeds and spooky action at a distance. But it involves absolute determinism in the universe, the complete absence of free will. Suppose the world is super-deterministic, with not just inanimate nature running on behind-the-scenes clockwork, but with our behavior, including our belief that we are free to choose to do one experiment rather than another, absolutely predetermined, including the "decision" by the experimenter to carry out one set of measurements rather than another, the difficulty disappears. There is no need for a faster than light signal to tell particle A what measurement has been carried out on particle B, because the universe, including particle A, already "knows" what that measurement, and its outcome, will be. https://en.m.wikipedia.org/wiki/Superdeterminism https://en.m.wikipedia.org/wiki/Superdeterminism I think it’s useful to be clear about the dividing line between demonstrated scientific results and our scientific intuitions, even when those intuitions are driven by observed patterns that have been reliable in the past. Intuitions are excellent drivers for formulating new theories and experiments, but it is epistemically dangerous to conflate beliefs and knowledge in our minds.